Improvements in the efficiency and size of DC—DC converters and/or regulators have resulted from advances in components, primarily semiconductors, and improved topologies. To a lesser extent, improvements in passive components, topologies, and soft switching schemes have also provided benefits. This is even true for linear regulators, where higher temperature components have reduced size and weight, and lower dropout voltage semiconductors and drive techniques have reduced the power loss.
One topology which has shown very high potential in limited applications is the Series Connected Boost Unit (SCBU), wherein a small DC—DC converter output is connected in series with the input bus to provide an output voltage equal to, or greater than, the input voltage. Because only a fraction of the power throughput is switched by the DC—DC converter, the overall system efficiency is very high. But this technique is limited to applications where it is sufficient that the output voltage is never less than the input voltage.
In the typical boost mode, the low voltage output of a small DC—DC converter is connected in series with the input voltage. The output voltage can then be adjusted from essentially equal to the input voltage up to the input voltage plus the maximum output voltage of the DC—DC converter. For example, a regulator for a 100 volt 1 kw bus could be constructed with a DC—DC converter having a 100 volt nominal input, and a 0 to 10 volt output and a 100 watt, 10 amp, rating. The configuration would allow regulating the output to up to 10% higher than the input, and yet only use switching and filtering components sized for 10% of the total rating, and it therefore offers greatly reduced size and power loss. The technique is very useful in situations where the input/output voltage ratio is relatively small, the output is always greater than the input, and isolation between the input and output is not required.
The change from linear to switching regulators was revolutionary, and the power loss and therefore size of regulators made a large step decrease. Since then the single most significant parameter determining converter efficiency and size, for a given frequency, has been the kva rating. The size of components to switch, transform, and filter is relatively independent of the topology, as the same amount of power is switched, rectified, and stored in filter components. The technique of partial power processing, wherein only a small fraction of the total output power is required to buck or boost the input to the desired output voltage, can significantly reduce the converter size and power loss.
For applications where output voltages equal to, or lower than, an input voltage are desirable, a buck converter is appropriate. However, it is desirable to have a converter design which provides a wide range of output voltages, both greater than, and lesser than, the input voltage. Further, high efficiency would be quite beneficial in such a design. The resulting buck-boost converter would thereby be quite useful in a wide variety of applications, including orbital or interplanetary applications.